1. Field of the Invention
The present invention relates to an optical pickup, and more particularly, to an error signal detecting apparatus of an optical pickup adopting hologram grating for suppressing the generation of offsets of focus and/or track error signals.
2. Description of the Related Art
In a high-capacity recording and/or reproducing optical pickup, focus and/or track error signals must be detected in order to perform a stable servo function. In general, in a recording/reproducing optical pickup, the path of the light emitted from a light source is changed by a light path changing means, e.g., a beam splitter, and the light is converted into focus light by an objective lens to then be incident onto a disk. The path of the light including an information signal reflected from the recording plane of the disk is changed by the light path shifting means to then be detected by a photodetector via a sensing lens and/or an astigmatic lens.
Referring to FIG. 1 schematically showing a conventional error signal detecting apparatus of an optical pickup, the light reflected from a disk (not shown) is received in a photodetector 6 having four light receiving regions which are photoelectrically converted independently, via a sensing lens 2 and/or astigmatic lens 4.
When the signals detected from the respective light receiving regions of the photodetector 6 are referred to as A, B, C and D, respectively, a focus error signal FES equals (A+C)-(B+D), and a track error signal TES, i.e., a push-pull signal, equals (A+D)-(B+C).
FIGS. 2A through 2C show a change in the light spots received in the photodetector 6 according to the distance between an objective lens (not shown) and a disk (not shown). FIG. 2A shows a light spot 8 formed on the photodetector 6 in the case where the distance between the objective lens and the disk is greater than the focal distance of the objective lens. FIG. 2B shows a light spot 10 formed on the photodetector 6 in the case where the distance between the objective lens and the disk equals the focal distance of the objective lens, that is, in an on-focus state. FIG. 2C shows a light spot 12 formed on the photodetector 6 in the case where the distance between the objective lens and the disk is smaller than the focal distance of the objective lens. Here, the illustration of FIG. 2A corresponds to the case where FES&lt;0, the illustration of FIG. 2B corresponds to the case where FES=0, and the illustration of FIG. 2C corresponds to the case where FES&gt;0.
In the above-described conventional error signal detecting apparatus, since the diameter of the light spot formed on the photodetector 6 is small, i.e., about 0.1 mm, the FES and TES signals become sensitive to deviations of the photodetector 6. Accordingly, focus and/or track offsets in which the FES and TES signals have values other than zero even at on-focus and on-track states may occur.
As the wavelengths of light emitted from a light source are changed for recording an information signal on a disk or the wavelengths vary according to the change in the temperature, a color aberration occurs at optical elements, In fact, in most of optical elements, an increase in the wavelength reduces the refractive index. Thus, a different light spot (14 in FIG. 3) from that being in an on-focus state in which there is no color aberration is received in the photodetector 6 even if the photodetector 6 is in an on-focus state, and the focus error signal is not zero.
As described above, even if a color aberration occurs due to a change in the wavelength, a servo drives an objective lens such that the FES signal becomes zero, thereby performing focusing. Thus, in an actual state, a defocused light spot is formed on a disk. If defocusing occurs during recording, the characteristics of a reproduced signal are deteriorated.